This is the cosmopolitan rhizocephalan barnacle Sylon hippolytes. Like its relatives on crabs, the parasite enters the host as a cyprid larva then grows into a network of tendrils invading every body system. When the barnacle is ready to reproduce it produces a sack-like externae that is filled with embryos. The externae is on the abdomen of the shrimp where normally the shrimp's embryos would develop.